Joachim Schwabe

Sea surface topography retrieved from GNSS reflectometry phase data of the GEOHALO flight mission

Sea surface topography observations are deduced from an airborne reflectometry experiment. A GNSS (Global Navigation Satellite System) receiver dedicated for reflectometry was set up aboard the German HALO (High Altitude Long Range) research aircraft. Flights were conducted over the Mediterranean Sea about 3500 m above sea level. A signal path model divided into large- and small-scale contributions is used for phase altimetry. The results depict geoid undulations and resolve anomalies of the sea surface topography. For the whole experiment 65 tracks over the Mediterranean Sea are retrieved and compared with a topography model. Tracks differ between right-handed and left-handed circular polarization. The difference, however, is not significant for this study. Precision and spatial resolution decrease disproportionately at low elevations. Eight tracks with centimeter precision are obtained between 11° and 33° of elevation. At higher elevation angles the number of tracks is significantly reduced due to surface roughness. In future such retrievals could contribute to ocean eddy detection.

New Antarctic gravity anomaly grid for enhanced geodetic and geophysical studies in Antarctica

Gravity surveying is challenging in Antarctica because of its hostile environment and inaccessibility. Nevertheless, many ground-based, airborne and shipborne gravity campaigns have been completed by the geophysical and geodetic communities since the 1980s. We present the first modern Antarctic-wide gravity data compilation derived from 13 million data points covering an area of 10 million km2, which corresponds to 73% coverage of the continent. The remove-compute-restore technique was applied for gridding, which facilitated levelling of the different gravity datasets with respect to an Earth Gravity Model derived from satellite data alone. The resulting free-air and Bouguer gravity anomaly grids of 10 km resolution are publicly available. These grids will enable new high-resolution combined Earth Gravity Models to be derived and represent a major step forward towards solving the geodetic polar data gap problem. They provide a new tool to investigate continental-scale lithospheric structure and geological evolution of Antarctica.

Regional Geoid Improvement over the Antarctic Peninsula Utilizing Airborne Gravity Data

We present an improved quasigeoid of the Palmer Land Region, Antarctic Peninsula, derived from recent aerogravimetry profiles provided by the British Antarctic Survey (BAS). Special focus is given to the treatment of the ice layer covering the bedrock topography, the latter one being regarded as the boundary surface. The remove-compute-restore technique (RCR) with least-squares collocation (LSC) and a point mass modeling, respectively, are applied and compared. In addition to previous studies, an alternative strategy regarding downward continuation has been introduced. Furthermore, the Residual Terrain Model (RTM) has been enhanced to incorporate the individual densities of water, ice and bedrock.

Regional geoid of the Weddell Sea, Antarctica, from heterogeneous ground-based gravity data

We present a geoid solution for the Weddell Sea and adjacent continental Antarctic regions. There, a refined geoid is of interest, especially for oceanographic and glaciological applications. For example, to investigate the Weddell Gyre as a part of the Antarctic Circumpolar Current and, thus, of the global ocean circulation, the mean dynamic topography (MDT) is needed. These days, the marine gravity field can be inferred with high and homogeneous resolution from altimetric height profiles of the mean sea surface. However, in areas permanently covered by sea ice as well as in coastal regions, satellite altimetry features deficiencies. Focussing on the Weddell Sea, these aspects are investigated in detail. In these areas, ground-based data that have not been used for geoid computation so far provide additional information in comparison with the existing high-resolution global gravity field models such as EGM2008. The geoid computation is based on the remove–compute–restore approach making use of least-squares collocation. The residual geoid with respect to a release 4 GOCE model adds up to two meters and more in the near-coastal and continental areas of the Weddell Sea region, also in comparison with EGM2008. Consequently, the thus refined geoid serves to compute new estimates of the regional MDT and geostrophic currents.

Subglacial Lake Vostok not expected to discharge water

The question whether Antarcticas largest lake, subglacial Lake Vostok, exchanges water is of interdisciplinary relevance but has been undecided so far. We present the potential pathway, outlet location, and threshold height of subglacial water discharge from this lake based on a quantitative evaluation of the fluid potential. If water left Lake Vostok, it would flow toward Ross Ice Shelf. Discharge would occur first to the east of the southern tip of the lake. At this location the bedrock threshold is 91 ± 23 m higher than the hydrostatic equipotential level of Lake Vostok. It is concluded that Lake Vostok is not likely to reach this level within climatic timescales and that no discharge of liquid water is to be expected. We show that in absence of the ice sheet the Lake Vostok depression would harbor a lake significantly deeper and larger than the present aquifer.

The evaluation of the geoid–quasigeoid separation and consequences for its implementation

The realization of precise height systems demands to assess the effect and necessity of approximations made to the pure theory. In this context, the formulas for the geoid–quasigeoid separation as presented by Flury and Rummel (J Geod 83:829–847, 2009) and further discussed by Sjöberg (J Geod 84:699–702, 2010) are reinterpreted. Starting from the fully topographically reduced gravity disturbance, a modification of the strict formulation of the downward continuation and the indirect effect according to Sjöberg (2010) is given. In practice any implementation of the formula requires approximations in order to realize the downward continuation of gravity along the plumbline with the help of density assumptions and a topography model. The significance of the individual contributors to a refined approximation, taking into account the indirect effect and the first-order gravity gradient, is elaborated in a numerical simulation for the example of the Himalaya region. Special focus is given on the sensitivity and convergency of the topography-induced terms with respect to the integration radius.